This invention relates in general to dissolved air flotation ("DAF") clarification of water. More specifically, it relates to a highly compact, variable capacity apparatus for treating municipal waste water to remove standard pollutants to within environmentally safe levels.
Water clarification is the removal of particulate contaminants suspended in water. Water clarification to remove total suspended solids ("TSS") is essential in any municipal water treatment system, as well as in sewage treatment. The water can be clarified by sedimentation or flotation of the particles.
Sedimentation facilities are in wide use, but their efficiency is relatively low. Their theoretical peak capacity is 0.5 gallons per minute per square foot. To handle large volumes of raw input water, treatment facilities using sedimentation tanks must be large and deep. There are obvious disadvantages in construction cost and land use. To increase capacity, a large area is covered with multiple, large concrete tanks arrayed side-by-side across a site. The weight of the tanks and the water they hold requires that they be set into the ground. Odors are usually open to atmosphere, increasing difficulty in siting a treatment plant.
Flotation techniques dissolve a few percent of air by volume in pressurized water, and then release the air in the form of microscopic bubbles that attach to the suspended particles. The bubbles carry the particles upwardly where they form a floating sludge. The particles are usually coagulated and flocculated by alum and polymers before the air bubbles are introduced. Flotation techniques are theoretically capable of clarification rates of 7.5 gallons per minute per square foot of flotation area, fifteen times the maximum theoretical rate for sedimentation techniques. U.S. Pat. Nos. 4,022,696; 4,377,485; 4,625,345; 4,184,967 and 4,931,175 to Milos Krofta describe earlier DAF type clarifiers operating in this general manner.
In the '696 clarifier, sold under the trade designations "SPC" and "Supracell", the flotation occurs in a circular tank. The raw water is fed into the tank via a central pipe to multiple outlets immersed in the tank which rotate about the tank. The inlet flow is oriented opposite to the direction of rotation of the inlet pipe and is at a velocity with respect to the rotation rate such that the raw water has a net zero velocity as it enters the tank. The raw water inlet flow assembly and a scoop for removing the floated sludge are mounted on a carriage that rotates about the tank. The scoop is preferably of the type described in applicant's U.S. Pat. No. 4,184,967. The rate of rotation is set so that the floated particles will reach the surface of the water held in the tank in the time of one rotation. A good degree of clarification can be achieved with each rotation of the carriage.
U.S. Pat. No. 4,377,485 discloses a later clarifier of the present application sold under the trade designations "SAF" and "Sandfloat", which utilizes the basic approach of the '696 invention, also operating with the net zero velocity principle, but also adding a second filtration stage with a set of wedge-shaped sand beds covering the bottom of the flotation tank. The water clarified by the flotation process flows through the underlying sand beds into a clarified water collecting chamber. A suction apparatus mounted on the carriage periodically backwashes each section of the filter in combination with a flow of backwash water introduced through the collection chamber. During clarification, when the flocs are formed, pressurized, aerated water is introduced to the open, lower end of the flocculation chamber via a manifold with multiple outlets. The bubbles carry the flocced particulates upwardly to form a floated sludge layer. The clarification rate is comparable to that of the '696 "Supracell" clarifier.
U.S. Pat. No. 4,626,345 discloses a clarifier, sold under the trade designations "SASF" and "Sandfloat-Sedifloat". It does not utilize the net zero velocity principle, but does employ a sand bed for two stage clarification. In this apparatus, the raw water enters a central cylindrical compartment, which acts as a hydraulic flocculator, and then flows over a dividing wall into a surrounding circular flotation tank. Aerated water is added via a fixed manifold. Rotating elements include a scoop remover for the floated sludge and a suction hopper that cleans the filter beds. The scoop is the same general type as used with the Supracell and Sandfloat units described above. This '345 SASF apparatus offers two stage clarification with a more compact unit and at a lower cost than the SAF clarifier.
While water clarification in the past has focussed primarily on the removal of suspended solids, in municipal waste water treatment it is also necessary to remove dissolved and colloidal organic particles as well as certain inorganic chemicals, particularly nitrogen and phosphorous-bearing compounds such as nitrites, nitrates and phosphates associated with organic waste, fertilizers and detergents. The organic particulates, if not treated, become septic and produce strong odors. They are controlled by biological oxidation. Their presence is usually expressed in terms of a biological oxygen demand ("BOD"). Other suspended contaminants are neutralized using chemical oxidation. Their presence is usually expressed in terms of a chemical oxygen demand ("COD"). Excess nitrates and phosphates are also a problem, among other reasons, because they stimulate the growth of algae in bodies of water that ultimately receive the treated effluent. Current U.S. Government standards require there be less than 30 mg/l of BOD's in the treated effluent, less than 30 mg/l of total suspended solids (TSS), and that the total of the TSS and BOD concentrations in the treated effluent be less than 15% of their initial concentration in the raw waste water.
Heretofore a variety of arrangements have been used to reduce the levels of these pollutants. One has been to add reactors in line with sedimentation tanks to reduce the BOD'S. The reactor can be as simple as a tank where air is added for bacteria to react with the organics. Suitable aerobic bacteria are fixed on a carrier immersed in aerated waste water. Known carriers include foams, plastics and sand. The LINPOR system is one example of this approach. It uses small cubes of a plastic foam material as the carrier. The cubes are added to an aeration tank where they tend to float. A large quantity (20% to 30% of the tank volume) must be used in order to get a good immersion and therefore a reliable interaction with the waste water. This approach increases the efficiency of the treatment system with no physical increase in its size, but it is very expensive. More generally, treatment with bioreactors is slow, costly, and requires additional treatments to control TSS and nitrogen and phosphorous-bearing compounds.
DAF clarifiers alone are excellent at removing TSS in a cost effective manner, but heretofore they have not been able to meet the aforementioned Government standards for BOD and combined BOD and TSS. For example, Supracell clarifiers are very efficient for removing TSS at a low cost, but Supracell units in general remove 40% to 60% of the BOD in the raw input water with the use of flocculation chemicals.
Known two stage clarifiers could also be used, but they also do not increase the clarification efficiency to a level that meets the Government standards. Moreover, any increased filtration due to a chemically enhanced removal of BOD's would add an extra load to the filter medium which would plug it, and in turn, would require increased backwashing. Further, two stage units are, in general, smaller, slower, and more costly to install and operate than single stage units such as the Supracell clarifier.
With a few notable exceptions, in conventional municipal waste water treatment to date DAF clarifiers have had a limited role, chiefly to remove TSS as part of treatment for peak flows such as storm sewer overflows during and immediately after a rainstorm.
It is therefore a principal object of this invention to provide a highly compact DAF clarifier system that is effective in lowering TSS and BOD.
A further object is to provide this advantage at a substantial cost savings, capital and operational, as compared to conventional sedimentation treatment facilities of comparable treatment efficiency and capacity.
Another object is to provide a biological treatment apparatus and method that can be used in conjunction with clarification to meet all U.S. Government clean waste water standards.
Still another object is to provide these advantages while also providing a capacity to handle peak flows as during rain storms or seasonal population increases.
Yet another object is to provide the foregoing advantages while also controlling odor and noise pollution.
A still further object is to provide these advantages while also producing a comparatively dry sludge output that can be readily handled among and/or further processed to control biological and heavy metal contaminants.